1. Field of the Invention
The present invention generally relates to digital exchanges, and more particularly to a multi-highway system of a digital exchange intended to facilitate a line extension operation and a function extension operation.
2. Description of the Related Art
Recently, there has been considerable activity in down sizing of digital exchanges by means of down sizing of various electronic devices and integration of circuits with a high density. As down-sizing of digital exchanges is facilitated, an increased number of lines (circuits) can be accommodated in printed-circuit board packages of a line system. Further, subscriber lines are digitized and multiplexed, and hence the number of usable lines tends to increase. Furthermore, recent digital exchanges cooperate with computers and hence provide highway advanced functions. Nowadays, various cards having various functions are available. Under these circumstances, it is desired that digital exchanges be flexible with extension of line-system cards and functional cards.
A digital exchange includes a time division switch which exchanges time slots. FIG. 1 shows a related digital exchange. The digital exchange shown in FIG. 1 includes highways 11, a time division switch 12, cards 13, card slots 14, card slot groups 15, and shelves 16. The cards 13 respectively have printed-circuit board packages having a plurality of lines and/or various circuits. The cards 13 are inserted into the card slots 14. Each of the shelves 16 includes six card slot groups 15. Each of the card slot groups 15 includes four card slots 14. The highways 11 are respectively assigned to the card slot groups 15.
Each of the highways consists of a pair of an up highway and a down highway. The up highway extends from the corresponding cards 13 to the time division switch 12, and the down highway extends from the time division switch 12 to the corresponding cards 13. One highway consists of, for example, 32 time slots; 30 out of the 32 time slots are used for speed communications. Hence, 30 channels are provided for each of the card slot groups 15. The remaining two time slots are used for communications control. The time slots for communications control are assembled in a multiframe structure and further in a supermultiframe structure. Control information conveyed via the time slots for communications control is transferred between a common control unit (not shown) and each of the cards 13, for example, every 32 ms. Further, one shelf includes 8 highways. Generally, two highways among the eight highways are used for conference communications and service tones in the time division switch 12. Hence, six highways (HW1-HW6), (HW9-HW14), (HW17-HW22), (HW25-HW30) are provided between the time division switch 12 and each of the shelves 16, respectively.
The time division switch 12 is controlled by the common control unit, and performs an exchange operation in which the time slots in the highways connected to the card slot groups 15 are exchanged with each other. Another time division switch is known which includes a spatial switch in addition to the time division switch 12.
FIG. 2 is a perspective view of an equipment cabinet in which the four shelves 16 shown in FIG. 1 are accommodated. In FIG. 2, "TDSW" denotes the time division switch 12. Further, the equipment cabinet includes a common control unit (CCU) and a power supply unit.
As has been described previously, one highway 11 is assigned to one card slot group 15. If three cards each having eight lines and one card having six lines are placed in one of the card slot groups 15, the four card slots 14 are efficiently used. However, if one card having 30 lines is placed in one of the four card slots 14 in one of the card slot groups 15, the remaining three card slots 14 cannot be used. In this case, the card slot group 15 being considered is not efficiently used. Further, there is another disadvantage in that it is necessary to provide other card groups in an extended shelf in order to accommodate other cards irrespective of whether or not the remaining three card slots are used. For this requirement, it is necessary to provide an equipment cabinet having a large size, taking into account a future extension of cards. Further, as a practical problem, it is necessary to provide connection cables between the equipment cabinet and a main distribution frame (MDF) beforehand. Hence, the structure shown in FIG. 1 is not suitable for down-sizing and needs an expensive initial production cost.